This invention generally relates to data storage devices and more particularly to protection of magnetoresistive heads and giant magnetoresistive heads and other sensitive devices from electronic over stress conditions.
Disc drives are data storage devices that store digital data in magnetic form on a rotating storage medium on a disc. Modern disc drives comprise one or more rigid discs that are coated with a magnetizable medium and mounted on the hub of a spindle motor for rotation at a constant high speed. Information is written to the magnetizable medium on the discs in a plurality of concentric circular tracks typically by an array of transducers mounted on the trailing end of xe2x80x9cheadsxe2x80x9d and carried by a radial actuator for movement of the heads relative to the discs. The read/write transducer, e.g. a magnetoresistive read/write head, is used to transfer data between a desired track and an external environment. During a write operation, data is written onto the disc track and during a read operation the head senses the data previously written on the disc track and transfers the information to the external environment.
Magnetoresistive (MR) transducers or heads, and more recently, giant magnetoresistive (GMR) transducers are typically formed by building layers by vapor deposition techniques on a head substrate. These layers which make up the transducers are extremely thin. As a result, they are extremely sensitive to static charge discharge events, and other electrical transients that can destroy or partially destroy the thin barriers between the element layers and/or between the layers and the substrate. Currently, only complete destruction of the layer integrity is detectable during the drive manufacturing process. Testing of the heads is performed periodically during the assembly stages to detect failed heads thus cutting down on the number of completed drives that fail. However, with the advent of smaller and smaller transducers, and the use of GMR heads, there has been an increased incidence of inoperable drive returns. These inoperative drives have passed the rigorous testing during manufacture and yet are still failing. Analysis of these drives shows that the heads failed not because of a catastrophic event, but because of weakening of the layer integrity. The cost of handling returned drives is high. Consequently, extreme care and expensive procedures have been taken to reduce such failures and testing frequencies have been increased. However, some of the failures continue to occur, even though the drives test satisfactorily during manufacture.
Accordingly, there is a need for device that can detect precursors to these electronic over stress conditions before they become significant and potentially damage the transducer.
Against this backdrop the present invention has been developed. The signals from the heads or between the preamplifier and the heads are monitored during the production stages. A number of electrical events may occur during the drive assembly process which are larger than normal yet smaller than that necessary to cause head failure at the time that the transient occurs. These signals have been detected and measured. The detection circuit in accordance with the present invention senses these signals on the line between the head and the preamplifier and monitors the energy content of the signals, and triggers an alarm condition if the energy content and or frequency of occurrence exceeds predetermined limits. Following manufacture, the detector circuit can be queried as to whether any alarm conditions have been sensed. If so, the drive can be rejected even though failure has not actually occurred or been indicated.
The circuit in accordance with the present invention monitors current signals between the preamplifier and the head and includes an amplitude section, a duration section, a frequency section, and an alarm section. The amplitude section has a comparator for detecting when a voltage transient exceeds a predetermined voltage value. The duration section measures the time period that the comparator has an output signal. The frequency section measures the frequency of occurrence of comparator output signals. Each of the sections provides an input to the alarm section which provides an alarm signal condition if the amplitude section magnitude is too high, or the duration times amplitude is too high, or the frequency of occurrences of events is too high.
These and various other features as well as advantages which characterize the present invention will be apparent from a reading of the following detailed description and a review of the associated drawings.